1. Field of the Invention
The present invention relates to a sheet feeding apparatus used in an image forming apparatus such as a copying machine, a printer and the like or in an image reading apparatus such as a facsimile, a scanner and the like.
2. Related Background Art
In the past, for example, in the sheet feeding portion of a copying machine or the like, sheet separation using a retard roller rotated in a direction opposite to a sheet feeding direction has mainly been used as sheet feeding means for preventing more than one sheets from being fed simultaneously (referred to as "double-feed" hereinafter).
Now, a conventional sheet feeding apparatus using a retard separation system will be briefly described.
FIG. 26 is a schematic side view of a sheet feeding apparatus of retard separation type comprising a sheet feeding roller (sheet pick-up roller) and a separating roller (refer to Japanese Patent Application Laid-open No. 3-18532, U.S. Pat. No. 5,016,866). This is referred to as a first earlier technology hereinafter.
As shown in FIG. 26, sheets S stacked on an intermediate plate 506 in a cassette 507 are lifted together with the intermediate plate 506 by a pressing arm 508 and a sheet pressing spring 505 to be always urged against a sheet feeding roller 501, thereby providing sheet feeding pressure.
Further, the sheet feeding roller 501 receives retard pressure from a separating roller 502. In this state, when the sheet feeding roller 501 is rotated in a sheet feeding direction, the sheet S urged against the sheet feeding roller 501 is picked up to reach a nip between the sheet feeding roller 501 and the separating roller 502. At this time, if a single sheet is pinched by the nip, the separating roller 502 is rotatingly driven in the sheet feeding direction by rotation of the sheet feeding roller 501 due to the presence of a torque limiter 503 formed integrally with a shaft of the separating roller, thereby feeding the sheet S.
However, if a plurality of sheets are pinched by the nip, the separating roller 502 is rotated with predetermined torque in a direction along which the double-fed sheets are returned, with the aid of the torque limiter 503, thereby preventing the sheet double-feed.
FIGS. 27 and 28 are schematic side views of a sheet feeding apparatus using a retard separation system constituted by a planetary gear mechanism (refer to Japanese Patent Publication No. 1-32134). This is referred to as a second earlier technology hereinafter.
As shown in FIG. 27, the sheet feeding apparatus utilizes a planetary gear mechanism comprising a sun gear 601, an intermediate gear 602, a planetary gear 603 and a connecting arm 604, and a sheet feeding roller 607 is connected to the planetary gear 603. Further, a separating roller 609 is connected to a drive shaft 606 through a torque limiter, and a pair of draw rollers 610 for feeding a sheet S at a speed higher than a speed at which the sheet feeding roller 607 feeds the sheets S are disposed at a downstream side of the sheet feeding roller 607 in a sheet feeding direction.
Now, an operation of the sheet feeding apparatus will be briefly described with reference to FIG. 28.
First of all, by rotating the drive shaft 606, the planetary gear 603 and the sheet feeding roller 607 are revolved in a direction indicated by the arrow A, with the result that the sheet feeding roller 607 abuts against an uppermost sheet S of a sheet stack contained within a sheet cassette. Further, in synchronous with such revolution, a lever 618 lifts an intermediate plate 623 on which the sheets are stacked toward the sheet feeding roller (in a direction indicated by the arrow G).
By this operation, the sheet S urged against the sheet feeding roller 607 is sent to a nip between the sheet feeding roller 607 and the separating roller 609, thereby effecting separation and feed of the sheet. Further, the sheet S left the nip enters into the pair of draw rollers 610, and the planetary gear mechanism and the sheet feeding roller 607 are returned to their original positions by transmitting a driving force of the pair of draw rollers 610 to the planetary gear mechanism through the sheet S. And, such operation is repeated.
Although two earlier technologies in the sheet feeding mechanism are shown, it is considered that such technologies can be improved in several points.
First, in the mechanism according to the first earlier technology, the sheets S stacked on the intermediate plate 506 within the cassette 507 are lifted together with the intermediate plate 506 by the sheet pressing spring 505 to be always urged against the sheet feeding roller 501. Thus, a sheet feed/separation condition greatly depends upon the pressure of the intermediate plate, with the result that an optimum sheet feeding area is limited in consideration of the pressure of the intermediate plate as a function.
In particular, since the pressures of the intermediate plate generated by the sheet pressing spring 505 vary with the number of sheets stacked within the cassette 507, the sheet feed/separation condition differs between a case where the cassette 507 is loaded up with the sheets and a case where a several number of sheets are stacked. Further, since the sheet S is always urged against the sheet feeding roller 501, the pressure of the intermediate plate always acts on the stacked sheets S. Thus, while the uppermost sheet S is being fed, a next or succeeding sheet S' is subjected to a feeding force due to friction between the sheets, with the result that the double-feed of sheet S' will easily occur.
In addition, even if the double-fed sheets are separated and tried to be restored, the sheets are pinched between the sheet feeding roller 501 and the intermediate plate 506 so that the double-fed sheets may not be restored smoothly.
Further, an allowable range of the appropriate sheet feed area is further limited in dependence upon the kind of sheet (for example, sheet having great coefficient of friction) and reduction of coefficients of friction of the sheet feeding roller and the separating roller due to wears of the sheet feeding roller and the separating roller, thereby worsening the stability.
Therefore, it is hard to say that this mechanism is a sheet feeding mechanism having high stability and high reliability.
Incidentally, in this mechanism, in order to make it so the double-feed is hard to occur and the double-fed sheets can easily be restored, the restoring force provided by the torque limiter 503 must be set to a greater value or the retard force of the retard spring must be decreased considerably or the feeding pressure provided by the sheet pressing spring 505 must be decreased considerably.
However, in many cases, slip between the sheet feeding roller 501 and the sheet and/or between the separating roller 502 and the sheet may easily be generated, with the result that the wear of the sheet feeding roller 501 and the separating roller 502 is accelerated, thereby reducing the service life of the sheet feeding roller 501 and the separating roller 502 greatly. As a result, the number of periodical replacing operation for worn parts is increased to increase the maintenance cost of the apparatus. Further, torque of driving force applying means (motor) must be increased, thereby making the apparatus expensive and increasing power consumption.
Further, when the restoring force of the torque limiter 503 is set to a greater value, in a space Z formed between a nip X (between the sheet feeding roller 501 and the separating roller 502) and an abutment area between the sheet feeding roller 501 and the intermediate plate 506, it is considered that the double-fed sheet (particularly, thin sheet having poor rigidity) may be buckled, thereby causing sheet jam.
In addition, when a pair of feeding rollers are provided at a downstream side of the sheet feeding roller 501 and the separating roller 502 in the sheet feeding direction, the pair of feeding rollers must draw the sheet S (always pressurized) from the intermediate plate 506 and the nip between the sheet feeding roller 501 and the separating roller 502, with the result that greater load will act on the pair of feeding rollers, thereby shortening the service life of the pair of feeding rollers.
Furthermore, since the intermediate plate 506 is always urged against the sheet feeding roller 501 by the sheet pressing spring 505, if this conventional technology is applied to a manual sheet feeding portion, when the operator sets the sheets, he must press the intermediate plate 506 down against the sheet pressing spring 505 to create a gap between the intermediate plate 506 and the sheet feeding roller 501 and insert the sheets into the gap.
This results in poor operability, so that an operator sheet setting failure may easily occur, which would lead to sheet jam and skew feed.
Next, in the mechanism according to the second earlier technology, the sheet feeding roller 607 is urged against and separated (retracted) from the stacked sheets S so that the intermediate plate 623 is pivotally moved by the lever 618 in upward and downward directions accordingly, thereby effecting pressurizing and releasing operations with respect to the sheet feeding roller 607. Namely, when the sheets S stacked on the intermediate plate 623 are fed, the sheets S are pinched by the sheet feeding roller 607 and the intermediate plate 623 from above and below.
Further, the retracting operation of the sheet feeding roller 607 and the lowering operation of the lower 618 are effected by utilizing the feeding force obtained when the fed sheet S is pinched between the pair of draw rollers 610. Accordingly, the stacked sheets S are pinched between the sheet feeding roller 607 and the intermediate plate 623 until a leading end of the fed sheet S reaches the nip of the pair of draw rollers 610.
Since the sheet feeding roller 607 is urged against the sheets S during the separating operation, the sheet are hard to be separated, and, further, since the leading end of the sheet S reaches the nip of the pair of draw rollers 610 during the urge, there is no timing for restoring the double-fed sheets.
In consideration of the sheet feed/separation condition, the sheet feeding mechanism according to the second earlier technology is the same as the sheet feeding mechanism according to the first earlier technology. Thus, as is in the first earlier technology, in this mechanism, since the appropriate sheet feed area is narrow, it is hard to say that it has high stability and high reliability. Further, the construction is very complicated and the number of parts is great.
Further, since the releasing of the pressure of the sheet feeding roller 607 against the sheet S and the revolving operation of the planetary gear mechanism and the sheet feeding roller 607 are effected by the feeding force of the pair of draw rollers 610, great feeding load acts on the pair of draw rollers, thereby reducing the service life of the draw rollers.
As a problem common to the above two earlier technologies, there is a problem that stability and reliability of sheet feeding and separating operations cannot be maintained adequately because the pressure of the intermediate plate influences the sheet feeding/separation condition. Further, during the separating operation, since the sheets stacked on the intermediate plate are urged against the sheet feeding roller, double-feed occurs easily and there is no timing for restoring the double-fed sheets, and, in dependence upon the kind of sheet, the sheet may be buckled to cause sheet jam.